The invention is grounded in the art of high-strength, high-modulus silicon carbide (SiC) monofilaments. Definitions:
1. A silicon carbide (SiC) monofilament is a monofilament that derives its properties from a bulk layer of essentially stoichiometric SiC.
2. An outer layer or surface layer is an extensive layer of material deposited on a SiC monofilament. A surface layer may be deposited directly on the bulk layer or on an intermediate buffer layer of material positioned between the bulk layer and the surface layer.
3. For purposes of this invention, the term "fine grain SiC" shall apply to polycrystalline SiC crystals with diameters or widths of less than about 200 nanometers (10.sup.-9 meters) and preferably less than between about 100-150 nanometers and a length of less than about 4 microns.
Typically, these monofilaments exhibit minimum tensile strengths in excess of 300,000 pounds per square inch (psi) and bending or Young's modulus in excess of 30 million psi.
The heretofore baseline 5.6 Mil Diameter monofilament has a tensile strength in the vicinity of 350-450 thousand psi and a bending modulus of 55-60.times.10.sup.6 psi.
The field of technology defined as high-strength, high-modulus monofilaments is unique and ultracritical to changes in structure or process of manufacture.
Persons skilled in the art of high-strength, high-modulus filaments have observed that it is frequently not possible to predict what effect changes in compositions, processes, feedstocks, or post-treatments will have on the properties of this unique family of filaments.
Boron nitride, boron carbide, titanium nitride, titanium carbide, and tungsten in combination with boron or silicon carbide monofilaments in specific applications have failed to provide a useful monofilament, though in each instance, the candidate material was chosen to enhance one or more properties of the high-strength monofilament.
One form of a silicon carbide surface layer on a silicon carbide monofilament would not protect the filament from degradation unless the coating had a critical cross-section profile. See U.S. Pat. Nos. 4,340,636 and 4,415,609.
Carbon cores heretofore required buffer layers deposited at critical specific temperatures. See U.S. Pat. No. 4,142,008.
Forms of carbon-rich silicon carbide surface layers which create and protect high tensile strengths of silicon carbide monofilaments were found to form ineffective metal matrix and resin matrix composite materials. See U.S. Pat. Nos. 4,340,636 and 4,415,609.
Recrystallization of fine grain structures occurs at one temperature. After 5 seconds of exposure to the temperature, the filament loses 50 percent of its strength. Exposure to a temperature on the order of 2 percent lower showed no subsequent degradation.
Critical crystal morphology of a carbon-rich region in a silicon carbide filament was found to improve machinability of silicon carbide reinforced composites. Proper claim structure defined the difference between a commercially viable and non commercially viable monofilament.
Other structural or manufacturing procedural features which were found to be of a critical nature are change in core composition and surface texture, the presence or absence of a buffer layer of specific compositions, impurities, and reactivity of a surface coating with matrix material.
SiC Monofilaments
The basic SiC monofilament comprises a core, generally of carbon or tungsten, about 0.5 to 1.5 mils in diameter upon which a thick stoichiometric SiC coating commonly called the bulk layer or bulk SiC is deposited.
To improve or tailor properties, intermediate layers and surface treatments in many forms have been tried and used. U.S. Pat. No. 4,340,636 referred to previously depicts what is considered the baseline commercial SiC monofilament commonly called SCS-2 monofilament It contains a carbon-rich intermediate buffer layer between the core and bulk layer.
U.S. Pat. No. 4,628,002 is directed to providing a silicon carbide monofilament having an improved transverse strain to failure ratio. The SiC monofilament is first provided with a layer of bulk SiC. There is deposited on the bulk SiC a thin layer of fine grain SiC and finally a surface layer or zone in which the Si/C ratio decreases then increases to provide a surface essentially of pure silicon. Each layer performs a critical function necessary to achieve the improved transverse strain to failure ratio.
Monofilaments containing solely a bulk layer, absent the discontinuity, deposited on a core are known to have been made. The crystal structure of these bulk layers is either unknown or significantly different from that disclosed and claimed herein.
The foregoing discussion is provided to emphasize that even small variations in structure and/or processing can lead to unexpected and/or critical improvement.